Many waste treatment systems and industrial processes have problems caused by foam that forms as the wastewater or process water flows through the system or process. Foam can occur in any aqueous stream that contains contaminants or additives that lower the surface tension of the stream. These materials are typically organic chemicals. They may be derived from natural chemicals (e.g., lignin, humic acid, tannin), waste chemicals, water treatment chemicals, process treatment chemicals, detergents, cleaners, products or byproducts of industrial processes, microbiological byproducts, etc. The system may also contain other materials that stabilize the foam after it has formed. Such materials include polymers, surfactants, suspended organic and inorganic solids, colloidal material, proteins, and microbiological organisms.
Foam problems frequently occur when a wastewater or process water stream is subjected to a unit operation that increases the total area of the system's gas/liquid interfaces. Such an increase occurs whenever a liquid is broken up into droplets or a gas is introduced into a liquid. Processes causing these effects include: mechanical processes (e.g., agitation, mixing, turbulent flow, pumping, aeration, gasification, reduction in pressure, increase in temperature), biological processes (e.g., fermentation, anaerobic digestion), and chemical processes (e.g., oxidation, recarbonation, gasification, distillation, solvent stripping, and reactions generating gas).
Foam in wastewater or process water is a problem for several reasons and can be detrimental to system processes (e.g., pumping, mixing, distillation, chemical reaction, heat transfer, evaporation, sedimentation, etc.). Foam can interfere with system sensors and controllers (e.g., level controllers, pH sensors, temperature sensors) and can adversely affect the quality and throughput of a product (e.g., crystal size and strength, holes in paper, and poor adhesion of coatings). Foam can also be a safety or health hazard (e.g., airborne bacteria and tank overflows), especially when aqueous media are highly acidic or caustic. It can also be an aesthetic problem (e.g., foamy discharge to receiving streams).
The Bayer process, a universally used process for producing alumina from bauxite (aluminum-containing) ores, experiences some of the most troublesome foam problems because of the inherent characteristics of bauxite (aluminum-containing) ore and the Bayer process. In the Bayer process, a finely ground bauxite ore slurry is digested in autoclaves at about 145-250° C. and 100-2000 p.s.i. in about 3-7 moles of sodium hydroxide solution to dissolve Al2O3·xH2O (x=1 or 3) as NaAlO2, for up to 2 hours. The digested slurry is discharged from the digesters, then cooled down to about 100° C. and brought to atmospheric pressure via a series of flash tanks. The solution is separated from solids by countercurrent washing with the aid of flocculants, and then by filtration processes to obtain solid-free solution, called the “pregnant liquor.” The pregnant liquor is supersaturated with NaAlO2, and is further cooled to about 70-80° C. and fed to precipitation vessels to precipitate NaAlO2 to Al2O3. 3H2O (alumina trihydrate crystals). The precipitation process is extremely slow, and may take up to 24-50 hours. The alumina trihydrate crystals are then calcined to remove binding water in the crystals to produce a final product, alumina (Al2O3).
During digestion in autoclaves, organic matter, in the form of complex cellulose and lignin substances associated with the bauxite ore are also extracted from the ore, and undergo oxidative attack in the caustic solution to form “humates” and the sodium salts of a variety of lower molecular weight organic acids. Since the Bayer process is a closed loop process, the solution in the system is continuously recycled to digestion where additional organic extraction from the bauxite occurs, with the organic impurity level building up in the solution stream and becoming quite significant, for example, up to 30 g/L of total organic carbon.
The organics in the solution stream cause significant foaming problems. Foaming usually occurs at any point after the digestion step where the pressure of the digested slurry is reduced to one atmosphere. The foaming of the solution is further aggravated by mechanical agitation and transfer of the solution from one vessel to the next. Foaming is especially a problem after separation of red mud, before and during the precipitation of alumina trihydrate. The foam poses a safety hazard due to its extremely caustic nature. Foam also complicates the heat control of the process, retarding heat loss, affecting crystal growth and quality, and reducing product yield and process efficiency. In light of the above safety, engineering and economic problems caused by Bayer process foam, improvements related to the control of foaming is a prime concern.
In the Bayer process, crystallization and precipitation of alumina prihydrates from sodium aluminate liquors is an important step toward the economic recovery of aluminum values. Bayer process operators optimize their precipitation methods so as to produce the greatest possible yield from the Bayer process liquors, while trying to achieve a given crystal size or crystal size distribution. It is desirable, in most instances, to obtain relatively large crystal sizes, since this is beneficial in subsequent processing steps required to produce alumina, alumina products, and/or aluminum metals. Chemical additives, such as flocculants and foam control reagents, may pose adverse effects on crystal growth, crystal size distributions, crystal strength and product purity. Therefore, any chemical additives to the process have to be carefully evaluated to avoid the creation of any undesirable side effects in the system.
There are several varieties of commercially available foam control products available for various industries. For example, block copolymers have been used as foam control reagents in the pulp and paper industry, food industry and paint and lacquer industry (U.S. Pat. Nos. 5,725,815, 5,538,668 and 4,836,951). A block copolymer is an effective foam controlling reagent, but its high cost limits its use in large quantities. Fatty alcohols and combinations of primary and alkoxylate alcohols are disclosed in U.S. Pat. No. 6,534,550. Their efficacy is demonstrated in the aqueous system from a paper making system. It is well known that polypropylene glycols with molecular weights in excess of about 1,000 are good antifoams because of their limited water solubility. The use of a water soluble polypropylene glycol with an average molecular weight of 200-600 in the Bayer process is disclosed in U.S. Pat. No. 5,275,628. Combinations of fatty alcohol and block copolymers used in the pulp and paper industry are disclosed in U.S. Pat. No. 5,562,862.
It is quite common that foam control reagents succeed only for a short period of time, with the activity of such products diminishing with time, so that it is frequently necessary to apply doses sequentially or at several empirically determined intervals. It would be desirable for foam control agents to suppress or eliminate foam for prolong periods of time. In the Bayer process, for example, it would be desirable if foam control agents are added to the precipitation vessels only at the beginning of the batch process, so as to effectively control foam for at least of 24 hours, or alternatively desirable if the activity of foam control agents could be maintained when the foam control agents move through consecutive precipitation vessels. Foam control products used to attenuate foaming are often more effective in neutral or weakly alkaline or weakly acidic media than in high caustic aqueous media, such as Bayer process solutions.
Effective antifoams should be insoluble, yet dispersible, in the foaming medium, be capable of controlling foam over a prolonged period of time, and have no negative impact on the down stream process. The present invention was developed in order to prevent or control the above described foaming problems.